Industrial automation systems generally include input and output circuits. It is desirable that the input circuit operates at high speed and be responsive to various type of signals from the input transducers. It is also beneficial for the industrial automation system to be resilient to operating characteristics of its input circuit and surrounding environment, e.g., electrical noise, that may be potentially adverse to its own operation.
A method to facilitate the industrial automation system's resiliency to these potentially adverse effects of the input circuit is to operably isolate the industrial automation system from the input circuit. One mechanism capable of such functionality is an opto-coupler. It is important to control the amount of current consumed in the circuitry associated with the input opto-isolator.
The amount of current consumed in the industrial automation system during an “on” state is the product of the input current and the input voltage. The input current design point depends on the current-transfer-ratio (CTR) of the opto-coupler. A typical CTR may range from 50 to 600%. In addition, the range of voltage for a discrete input from an IO point also has a broad range. The combination of these two broad ranges of input current and voltage presented to the opto-coupler may be intolerable.
This invention is directed to solving these and other problems.